This invention relates generally to etching processes and in particular to a method for controlling the profile of an opening etched in a layer of material.
In the fabrication of semiconductor integrated circuit devices vias or openings are formed in an insulating layer prior to metallization to provide contacts to underlying regions. It is preferable that these openings have a rounded profile in order to minimize the possibility of defects in the overlying metal layer. One problem is a step-coverage defect, which sometimes occurs when a metal layer is formed over an opening having a steep profile and causes a discontinuity in the conductor formed by the metal layer. Such steep openings, that is, openings having nearly vertical sidewalls, typically occur when an insulating layer is anisotropically etched, for example by a plasma or reactive ion etching process.
The insulating layer via profile becomes more important as the number of metal interconnection layers increases and the thickness of each metal layer decreases.
One method for providing an opening having a sloped profile is to form a predetermined slope in the sidewalls of the openings in a mask layer overlying the insulating layer to be etched. The sidewall profile in the mask layer, typically a photoresist, is then transferred to the opening in the insulating layer during the etching process. A disadvantage is that this method requires an extra high temperature bake step to form the desired opening profile in the mask layer. This step to obtain a predetermined slope in the mask layer is not easily controlled, thus resulting in an etch profile that is difficult to repeat from wafer to wafer.
Another method of providing a sloped sidewall profile during anisotropic plasma or reactive ion etching is to vary the ion bombardment energy. However, this requires a complex triode or a flexible diode reactor and it is often difficult to precisely control the profile.
The prior art teaches various methods of tailoring the reactive etchant species used in plasma etching to achieve a particular etch rate and selectivity relative to the layer being etched, the underlying layer and the photoresist mask layer. For example, U.S. Pat. No. 4,174,251 to Paschke describes a two step etching process for a low pressure plasma reactor wherein a silicon nitride layer is etched through a hydrocarbon photoresist mask without destroying the mask layer. The process includes a pre-etch step using a high plasma power level and a 95:5 CF.sub.4 :O.sub.2 etchant gas to etch halfway through the silicon nitride layer, followed by a main etch step at a lower power level, using a 50:50 CF.sub.4 :O.sub.2 etchant gas to etch the remainder of the silicon nitride layer.
U.S. Pat. No. 3,940,506 to Heinecke discloses a method of adjusting the concentration of a reducing species, such as hydrogen, in a plasma to control the relative etch rates of silicon and silicon dioxide or silicon nitride, particularly for use in a low pressure plasma reactor. Hydrogen is used to control the selectivity and may be added to the CF.sub.4 etchant gas mixture by using a partially fluorine substituted hydrocarbon such as CHF.sub.3.
U.S. Pat. No. 4,324,611 to Vogel et al. describes a method for tailoring a reagent gas mixture to achieve a high etch rate, high selectivity and low breakdown of photoresist in a single wafer, high power, high pressure reactor. The disclosed reagent gas mixture includes a primary etching gas consisting of a pure carbon-fluorine, and a secondary gas containing hydrogen to control the selectivity of the etch. A tertiary gas containing helium may be included to prevent the breakdown of the photoresist mask layer. In one embodiment for plasma etching silicon dioxide or silicon nitride overlying silicon, the primary gas is C.sub.2 F.sub.6 and the secondary gas is CHF.sub.3.